The ozone water obtained by dissolving ozone gas in water exhibits excellent disinfection, deodorization and bleaching actions on account of the strong oxidizing power of ozone. In addition, because ozone gas undergoes auto-decomposition to oxygen (gas), which is harmless, with the passage of time and causes no residue problem, it is attracting attention as an ecofriendly disinfectant, cleaner and bleach, for instance.
In order to insure a positive disinfectant, cleaning and bleaching effect, the dissolved ozone gas concentration of ozone water must be high enough but since ozone gas is not only sparingly soluble in water by nature but has the disadvantage that increasing the ozone gas concentration frivolously would reduce the efficiency with which ozone is utilized and, hence, lead to a waste of the material, it is of great significance to control the ozone gas concentration suitably in the production and use of ozone water.
The conventional ozone water production and/or purification system which is generally employed is a bubbling system in which ozone gas is bubbled into water or an ejector system involving the use of an ejector.
The bubbling system has the disadvantage that it requires a gas-liquid separation column for removal of bubbles. Moreover, although the finer the mist of ozone gas bubbled is, the higher is the rate of dissolution attained, the gas-liquid separation is a time-consuming process. Thus, in the bubbling system, the efficiency of dissolution of ozone gas in water is very low and the dissolved ozone gas concentration can hardly be controlled appropriately.
Meanwhile, in the manufacture of semiconductor or liquid crystal devices, a stripping operation for removing the used photoresist is carried out frequently in the photolithographic process. In this photoresist stripping operation, the use of chemicals in large quantities and the high temperature required for photoresist removal impose a considerable burden on clean-room air conditioning and amidst the mounting interest in the protection of environment, a resist stripping technology using ozone water as a substitute for the conventional chemicals is attracting attention.
In connection with the above resist stripping technology involving the use of ozone water, the relationship of ozone water concentration to the resist striping rate was investigated and it has by now been elucidated that increasing the ozone water concentration contributes much to an increased stripping rate (Collection of Papers Read Before the 8th Annual Research Meeting of Japanese Ozone Association, pp. 14-16, Mar. 3, 1998).
Furthermore, although the cleaning of semiconductor wafers and other substrates has been conventionally carried out by the RCA process using sulfuric acid, ammonia or hydrochloric acid, the use of ozone water for this cleaning has been studied of late from ecological and other considerations (Mitsubishi Electric Technology Vol. 173, Nov. 4, 1999).
Japanese Kokai Publication Hei-7-213880 discloses an ozone gas-permeable membrane in the form of a porous hollow tube. However, in the case of a porous ozone gas-permeable membrane, it may not be possible to preclude contamination of ozone water with metal particles finding their way into the ozone gas during gas production insofar as an ozone generator of the silent discharge type is employed.